1. Field of the Invention
This invention relates to drive control apparatuses for electric injection molding machines that are used for injection molding of resin.
This application is based on Patent Application No. Hei 9-184194 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
In general, resin molding is performed as follows:
The screws inject melted resin into cavities whose interior walls contribute to formation of exterior shapes of molded goods, wherein the predetermined pressured is applied to the injected resin.
In the electric injection molding machine, servo motors are normally provided for the injection device used for injection and the closing device used for closing (or clamping) of the cavity respectively. However, as the scale of the injection molding machine becomes large, an amount of required drive torque becomes large as well.
The electric injection molding machine of the present technology uses the synchronous ac servo motors in general. However, there is a limit in maximum torque which can be produced by one servo motor. For this reason, the electric injection molding machine of the large scale comprises multiple servo motors, which are subjected to synchronous drive to produce the "required" torque.
Next, a description will be given with respect to examples of drive control apparatuses for controlling servo motors employed in the electric injection molding machine with reference to FIG. 9 and FIG. 10.
FIG. 9 shows a first example of the drive control apparatus for controlling multiple servo motors. In FIG. 9, a rectifier 10 and a capacitor C construct an inverter, which converts three-phase alternating current supplied from a three-phase alternating current power source E to direct current. Two transistorized inverters 12 and 12' are connected to output terminals of the rectifier 10.
The transistorized inverter 12 comprises three sets of switching circuits, which are provided to supply predetermined currents with respect to three phases (i.e., phases U, V and W) of a servo motor M1. Herein, each of the switching circuits is configured by two transistors and two diodes. For example, the switching circuit supplying current for the phase U of the servo motor M1 is configured by two transistors T.sub.A, T.sub.B, which are connected in series between the output terminals of the rectifier 10, and two diodes D, D which are connected in series between the output terminals of the rectifier 10. Herein, a connection point between the transistors T.sub.A, T.sub.B is placed in a conducted state with a connection point between the diodes D, D. Other switching circuits provided for the other phases V and W are configured similar to the aforementioned switching circuit provided for the phase U. Thus, those three switching circuits are connected to the servo motor M1 with respect to the phases U, V and W respectively.
The transistorized inverter 12' has roughly an identical configuration of the aforementioned transistorized inverter 12. So, the description regarding details of the transistorized inverter 12' is omitted.
Incidentally, the transistorized inverter 12 is configured using six transistors T.sub.A to T.sub.F, while the transistorized inverter 12' is configured using six transistors T.sub.A ' to T.sub.F '.
A transistor PWM control circuit 14 (where "PWM" is an abbreviation for "Pulse-Width Modulation") is connected to bases of the aforementioned twelve transistors in total.
The servo motor M1 is equipped with a position detector 18 for detecting rotating position of a rotor as well as current detectors 20, 22 for detecting currents flowing across the phases U and W respectively. Herein, the position detector 18 produces a position detection value S, while the current detectors 20, 22 produce current detection values I.sub.U, I.sub.W respectively. Those values are input to the transistor PWM control circuit 14.
The transistor PWM control circuit 14 changes on/off times and time intervals of the transistors T.sub.A to T.sub.F and T.sub.A ' to T.sub.F ' in response to a speed instruction signal Vo output from a controller 16 and the position detection signal S input thereto. Thus, it controls amounts of currents flowing across the phases of the servo motors M1, M2 so as to control rotation speeds of the servo motors M1, M2.
As described above, in the first example of the drive control apparatus, a single transistor PWM control circuit 14 controls the two transistorized inverters 12, 12' in response to the speed instruction signal Vo given from the controller 16 and the position detection value S which is detected with regard to one servo motor M1. Thus, it is capable of driving the two motors M1, M2 in a synchronized manner.
FIG. 10 shows a second example of the drive control apparatus for controlling multiple servo motors. In FIG. 10, a reference symbol "30" designates a main body of a control unit that is configured by a digital controller 32 and a motor interface 34. The digital controller 32 is connected to a master servo amplifier 38 by means of a pressure-control printed-circuit board (I/F) 36. The master servo amplifier 38 drives a servo motor 42. In addition, a slave servo amplifier 40 drives a servo motor 44. That is, an injection member 46 is driven by the servo motors 42 and 44.
A load cell 48 detects pressure which is applied by the injection member 46. Then, a detection value of the load cell 48 is input to the pressure-control printed-circuit board 36.
The servo motors 42 and 44 are respectively equipped with speed detectors (TG) 50 and 52, which in turn detect rotation speeds of the servo motors 42 and 44 respectively. Detection results of the speed detectors 50 and 52 are input to the master servo amplifier 38 and the slave servo amplifier 40 respectively.
Instruction data (or command data) are input to the digital controller 32 in advance to determine "preset pressure", by which the master servo amplifier 38 drive both of the servo motors 42 and 44 so that the injection member 46 moves forward and backward. Namely, the second example of the drive control apparatus is designed as follows:
Based on control signals output from the master servo amplifier 38, it performs speed control on the servo motors 42 and 44 in such a way that their rotation speeds are adjusted uniformly in an electrical manner.
The aforementioned examples of the drive control apparatuses suffer from problems as follows:
The aforementioned first example of the drive control apparatus shown in FIG. 9 is designed in such a way that the two transistorized inverters 12 and 12' are provided for the servo motors M1 and M2 respectively, wherein they are driven by a single transistor PWM control circuit 14 to control operations of the servo motors M1 and M2. For this reason, the first example requires specially designed configurations for the servo motors M1 and M2 as well as the their control devices. In addition, those configurations should be complicated ones.
In addition, the second example of the drive control apparatus shown in FIG. 10 is designed to perform speed control by merely adjusting rotation speeds of the servo motors 42 and 44 in an electrical manner. For this reason, the second example is somewhat weak against disturbances. So, hunting easily occurs in control of the second example.